This invention relates to a method of optimizing a control scheme of a refrigeration system during steady state operation. In particular, the method is directed to a container refrigeration system.
A refrigeration system attached to the container cools the goods within the container to a target temperature. In the steady state regime system cooling capacity must be matched to the required refrigeration load in order to maintain tight temperature control. At any given point in time, the refrigeration system cooling capacity is determined by the system operating conditions, which in turn depend on the ambient temperature, the temperature inside the refrigerated container and the characteristics, and mode of operation, of the compressor and other refrigeration system components, such as suction modulation valve, heat exchangers, etc. On the other hand, the required refrigeration load is mostly a function of ambient temperature, temperature in refrigerated space, product respiration load and container size and insulation characteristics.
Once the system has reached, or at least approached, the target temperature, it is necessary to continuously adjust the capacity of the refrigeration system, while maintaining operation within a predetermined range of the target temperature. In the past, the controls associated with the refrigeration systems have not been sophisticated enough to achieve the reduced capacity while maintaining reliable and energy efficient system operation with accurate temperature control. Instead, generally, the refrigeration systems have simply on/off-cycled the compressor. Despite the simplicity and ease of on and off control, many refrigeration systems cannot effectively use this method due to the inability to maintain a tight temperature control in the refrigerated space. Further, this method has sometimes had reliability problems with electric motors and compressors caused by mechanical and/or electrical overloading due to the on/off cycling. Finally, in applications wherein there are widely varying load conditions, this method results in poor energy efficiency.
The prior art tried to achieve tight temperature control using throttle valves in the suction lines, and additional components such as compressor unloaders, bypass schemes, split coils, variable speed drives, multiple compressors, and various operations of the several systems to achieve the reduced capacity. However, these techniques often proved to be costly or unreliable thus there has still been a desire to achieve a more sophisticated method of controlling the capacity to optimize steady state control with respect to temperature control accuracy, energy efficiency and reliability.